The desirability of sustained release drug formulations has long been a goal in the pharmaceutical industry. Sustained release systems solve many of the problems associated with conventional drug delivery systems, e.g., pills. For example in conventional drug delivery systems administration of the drug is given frequently and results in high variability in circulating drug levels during the course of treatment. The concentration of the drug increases to therapeutic concentrations after administration, but in some instances the concentration rises above the minimal therapeutic level reaching the toxic threshold. After a relatively short period of the drug concentration decreases via metabolization or excretion to levels that are no longer therapeutic.
In order to achieve constant levels of drugs and avoid the inefficiencies of the drug concentration peaks and valleys the drugs should be released from a delivery system at a rate that does not change with time (so called zero- order release). Preferably, the initial dose of a drug is the therapeutic dose, which is maintained by the delivery system. Examples of a current sustained drug delivery system include the reservoir systems which consist of tubes, fibers, laminates, or microspheres. In these systems, a drug reservoir is coated in a rate-controlling membrane. Drug diffusion across the membrane is rate limiting and is constant (zero order) as long as the membrane's permeability does not change and as long as the concentration of drug in the reservoir is constant.
In matrix systems drugs are dispersed through a matrix and are released as the drugs dissolve and diffuse through the matrix. A drug is released from the outer surface of the matrix first, this layer becomes depleted, and a drug that is released from further within the core of the device must then diffuse through the depleted matrix. The net result is that the release rate slows down and thus it is very difficult to maintain constant and consistent release. Another type of device are the polymeric devices that contain a polymeric material that is permeable to the passage of the drug. Although it is possible to choose from among a large variety of polymeric materials, in practice only a small number of polymers have been shown to function satisfactorily as a release determining outer layer of the reservoir. Sustained release systems that release two or more active substances over a prolonged period of time are extremely useful for certain applications, for example, the in fields of contraception and hormone replacement therapy.
Ring-shaped vaginal drug delivery devices (“vaginal rings”) are well known in the art. Such devices are designed to deliver a relatively constant dose of drug to the vagina, usually over a period of weeks to months. Typically, they are made of a silicone elastomer and contain a drug released by diffusion though the elastomer. Vaginal rings have been developed for delivering steroids to treat post-menopausal vaginal conditions, as well as for contraception and hormone replacement therapy. Women generally prefer vaginal rings to oral delivery for several reasons, particularly their convenience, privacy, long-term delivery capacity, and effectiveness. Vaginal rings provide a regulated dose of drug with minimal involvement or attention by the user. They also avoid the first pass of orally administered drugs through the liver, where appreciable portions of the daily dosage of some orally administered steroids are degraded.
U.S. Pat. No. 4,292,965 (Nash et al.) and U.S. Pat. No. 4,822,616 (Zimmermann et al.) disclose two-layered vaginal rings. The rings comprise an inner drug-free supporting ring, a middle layer comprising a drug, and an outer drug-free layer. All three layers preferably comprise a silicone elastomer. However, the use of silicone elastomers is now generally considered to be unsafe, and is no longer the material of choice.
In addition to single drug delivery, vaginal rings have been developed for simultaneous release of multiple drugs over a prolonged period of time. For example, U.S. Pat. No. 3,995,633 (Gougen) and U.S. Pat. No. 3,995,634 (Drobish) disclose vaginal rings comprising separate reservoirs containing different active substances, wherein the reservoirs are arranged in holders. U.S. Pat. No. 4,237,885 (Wong et al.) also discloses a multi-reservoir device, in which spacers are used to divide a tube or coil into portions, wherein each portion is filled with a different active substance in a silicone fluid and the two ends of the tube are subsequently connected to form a ring. Patent Publication WO 97/02015 (Groenewegen et al.) discloses a two-compartment device, wherein one compartment has a core, a medicated middle layer and a non-medicated outer layer, and a second compartment having a medicated core and a non-medicated outer layer. However, the active substances in these multi-compartment or multi-reservoir devices typically diffuse through the walls of the tubes, thus allowing drug interactions, particularly during prolonged storage. Interactions between the drugs often results in degradation or inactivation of at least one of the drugs, and thus variations in the pre-set fixed release ratio between the drugs over time.
The device described in U.S. Pat. No. 4,596,576 (di Nijs) was designed to overcome the diffusion problem associated with multi-compartment devices. di Nijs discloses a two-compartment vaginal ring wherein each compartment comprises a reservoir filled with a different active substance. However, to prevent diffusion and maintain a constant release ratio between the various active substances over time, the drug compartments are separated by impermeable inert stoppers, formed of glass, gold or silver. Although the stoppers effectively prevent diffusion of active substance between reservoirs, the device is complicated and expensive to manufacture.
The device described in U.S. Pat. No. 5,989,581 (Groenewegen) was also designed to overcome the diffusion problems associated with existing delivery systems, as well as to provide a less complicated and cheaper device for intravaginal delivery of multiple active substances. The patent discloses a ring-shaped drug delivery system for the simultaneous release of a progestogenic steroid compound and an estrogenic steroid compound, reportedly in a fixed ratio over a prolonged period of time. The drug delivery system has a compartment comprising a thermoplastic polymer core containing the mixture of the progestogenic and estrogenic compounds and a thermoplastic polymer skin. However, like other known vaginal devices, the Groenewegen device suffers from its own inherent limitations. In general, the release per unit time of a drug is determined by the solubility of the active substance in the outer layer (wall) of polymeric material and by the diffusion coefficient of the active substance in the wall. Thus, the choice of the outer layer material of the reservoir largely determines the release ratio of the active substances contained in the reservoir. Unfortunately, only a few polymers are capable of functioning satisfactorily as a release-determining outer layer of the reservoir. Finding the appropriate polymer for a particular drug or drug combination can be difficult. Moreover, the reservoir material must be capable of taking up a large amount of the active substance or substances in order to provide an adequate supply of the substances to the outer wall. Meeting these challenges is problematic, if not impossible, and must be addressed for each new drug or drug combination.
An intravaginal drug delivery device which can release two or more active substances in a substantially constant ratio to one another over a lengthy period of time would be extremely useful for certain applications. For example, in the field of contraception and in the field of hormone replacement therapy, extensive use is made of the simultaneous administration of an agent having a progestogenic activity and an agent having an estrogenic activity, preferably in a substantially constant ratio.
Drug delivery systems and methods for contraception and for treating female reproductive disorders using combination therapy have been developed. For example, U.S. Pat. No. 4,762,717 (Crowley, Jr.) discloses a delivery system for the continuous delivery of LHRH compositions in combination with sex steroid for use as a contraceptive. U.S. Pat. No. 5,130,137 (Crowley, Jr.) discloses a similar delivery system for the treatment of benign ovarian secretory disorders.
Despite these significant advances in the field, a need exists for an improved method for contraception and treatment of female reproductive conditions and disorders, such as treatment of benign ovarian secretory disorders and post-menopausal hormone replacement therapy. In particular, a method that uses a hormone replacement steroid and/or a LHRH and/or its analogues (either agonists and/or antagonists), and that can be administered in a safe, physiologic, and convenient manner, would be highly desirable. Moreover, a need exists for an improved drug delivery system for the simultaneous release of LHRH and sex steroids or sex steroid modulators, particularly a system which releases the LHRH and steroids in a substantially constant ratio over a prolonged period of time, and which is easy and inexpensive to manufacture.
Additionally, constant and reliable delivery of drugs or combinations of drugs over long periods of time would be useful in a wide variety of applications, including treatment or prevention of AIDS/HIV, atherosclerosis, various cancers, cardiovascular diseases, hypertension, toxemia of pregnancy, seizures degenerative neurological disorders, diabetes, hematological disorders, addictions, and obesity and eating disorders, to name a few. Unfortunately, in addition to not always providing satisfactory release, release ratio or release term in some cases, all currently available intravaginal drug delivery devices suffer from being relatively complicated, making them expensive to manufacture. Thus, a need exists for an improved drug delivery device for the simultaneous release of multiple drugs, particularly a device which releases the drugs in a substantially constant ratio over a prolonged period of time, and which is easy and inexpensive to manufacture.